Multiple-input-multiple-output (MIMO) systems represent an advance in wireless communication. MIMO employs multiple antennas at the transmitting and receiving ends of a wireless link to improve the data transmission rate while holding radio bandwidth and power constant.
A MIMO transmitter transmits an outgoing signal using multiple antennas by demultiplexing the outgoing signal into multiple sub-signals and transmitting the sub-signals from separate antennas. MIMO exploits multiple signal propagation paths to increase throughput and reduce bit error rates. Each sub-signal reflects off the local environment along its associated signal propagation paths. The spatial richness of the local environment is a function of the uniqueness and distinctness among the different associated signal propagation paths. While multiple signal propagation paths cause interference and fading in conventional radios, MIMO uses these multiple signal propagation paths to carry more information than conventional radio transmissions.
FIG. 1 illustrates a basic MIMO wireless link 10, where the transmitter 20 has Mmax transmitting antennas 21 (21-1 . . . 21-m), and the receiving station 30 has N receiving antenna 31 (31-1 . . . 31-n), the number of transmitters active at a given moment is M, such that M<=Mmax. A scattering environment 50 with some degree of spatial richness, or statistical independence of fading coefficients, exists between the transmitter and receiver. The channel matrix H represents the channel connection characteristics (or impulse response) between the transmitting and receiving antennas, 21 and 31, respectively.
Most improvements on multiuser MIMO systems have been directed to single cell environments in which one base station serves several users. However, in multiple cell environments, capacity gain is degraded.
Moreover, most prior network MIMO algorithms that were designed to support multiple users and improve capacity gain assumed that all base stations in a multi-cell environment have to share all the data messages to be transmitted to each user. This assumption is difficult to implement.